The present invention generally relates to a data terminal equipment, and more particularly to a data terminal equipment which uses a hierarchical protocol based on an OSI standard reference model. The present invention further relates to a data transmission control method suitable for the above-mentioned data terminal equipment.
Recently, there is a trend that a data terminal equipment which uses a digital line as a transmission line uses, as a transmission protocol, a hierarchical protocol based on an OSI (Open Systems Interconnection) standard reference model in order to establish data transmission between devices of different types. The hierarchical protocol consists of first to seventh layers arranged in this order from the lowermost layer. The first layer is a physical layer which controls a physical medium. The second layer is a data link layer that ensures communication up to an adjacency open type system. The third layer is a network layer which ensures communication up to an end open type system. The fourth layer is a transport layer which presents a transparent transmission line. The fifth layer is a session layer which controls conversation. The sixth layer is a presentation layer which unifies information presentation styles. The seventh layer is an application layer which presents system management and protocols for users. The OSI reference model is described in "DATA TRANSMISSION TECHNIQUES FOR PRACTITIONERS", NTT Suzuka Electric and Communication School Data Transmission Working Group, for example.
Conventionally, a high level data link control (HDLC) procedure is widely used as a protocol for the second layer, i.e., the data link layer in the hierarchical protocol. Particularly, when a public digital network is used as a transmission line, link access procedure balanced data link protocols defined in the CCITT recommendation X.21 or X.75 are used as HDLC procedures. Hereinafter, the link access procedure balanced data link protocol is simply referred to as LAPB data link protocol.
The LAPB data link protocol is used for data transmission of point-to-point type. In the LAPB data link protocol, data to be transmitted is arranged to a predetermined frame format, and is transmitted for every frame. A receiver determines whether a data error is contained in data for every frame. When a data error is found, the receiver can request the transmitter to retransmit the frame related to the found data error. Thus, the receiver can obtain data having no data error.
The receiver can put together consecutive frames which have continuously been received and subject the consecutive frames to the error checking procedure. According to the LAPB data link protocol, a maximum number of frames which can continuously be received is limited. A maximum of frames is seven in a standard mode, and 127 in an extended mode. In the following description, an LAPB modulo-8 data link protocol indicates the case where a maximum of frames which can consecutively be received is seven, and an LAPB modulo-128 data link protocol indicates the case where a maximum of frames which can consecutively be received is 127.
On the other hand, recently, integrated services digital networks (hereinafter simply referred to as ISDNs) are being built. In ISDNs, a link access procedure for D-channel is used as a data link protocol for a signal channel (D channel) for performing a line control such as a call setting up procedure. Hereinafter, this link access protocol for D-channel is simply referred to as LAPD data link protocol. The LAPD data link protocol is based on the LAPB data link protocol and is obtained by adding new functions necessary for ISDNs to execute the LAPB data link protocol. The LAPD data link protocol can be applied to D channel, and further will be applied to information channels of ISDNs (B channels) and data link protocols used in the existing public digital networks.
From the above-mentioned viewpoints, there is a possibility that protocols of the three different types, that is, the LAPB modulo-8 data link protocol, the LAPB modulo-128 data link protocol and the LAPD data link protocol may be used together as data link protocols for information channels. In other words, data terminal equipments based on the different data link protocols may be used together.
ISDNs are intended to integrate functions provided by conventional public switched telephone networks (PSTNs), circuit switched public data networks (CSPDN) and packet switched data networks (PSDNs) in the future and will be used in place of those networks. However, it is estimated that it takes long time the conventional networks to be replaced with IDSNs. As a result, ISDNs, PSTNs, CSPDNs and PSDNs by be used together until the replacement is completed.
It is basically necessary for the data terminal equipment itself to have an ISDN-based transmission function in order to be coupled to ISDN. In actuality, it is possible to connect a conventional data terminal equipment to ISDN by use of a terminal adapter which realizes ISDN transmission functions. The terminal adapter realizes a signal control and a packet communication control which use the signal channel (D channel) of ISDN, and low-level layer functions on information channel thereof.
Generally, a data terminal equipment has transmission functions provided by high-level layers, which are used when connected to existing networks. Therefore, even when the data terminal equipment is connected to ISDN through the terminal adapter, the high-level layer transmission functions are used.
As data terminal equipments coupled to digital line networks, there is a facsimile machine or a telematic terminal such as a Japanese teletex device. When a group-4 facsimile machine uses, as a transmission line, CSPDN or PSDN, the transmission function defined in th CCITT recommendation T.70 is used.
On the other hand, according to the CCITT recommendation T.90, the transmission function defined in the ISO standard ISO8208 is a standard function in ISDN's. The CCITT recommendation T.90 sanctions using the CCITT recommendation T.70 transmission function as an optional transmission function.
As described above, there are plural protocols for each layer With respect to a data terminal equipment connected to ISDN. Therefore, it is impossible to perform data transmission between data terminal equipments which have the same terminal function but the different transmission functions. This is inconvenient.